feat(gen-ai): add gen_ai.memory.store.name attribute

Add optional human-readable name for memory stores, separate from
the machine-generated store.id. This is opt-in on all memory spans.

Supported by 3 of 6 frameworks:
- AWS Bedrock: name field on CreateMemory
- Azure AI Foundry: name param on memory_stores API
- Letta/MemGPT: label field on Block

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nagkumar91